The present invention relates generally to digital data communication links and more particularly to a repeater for use in serial data communication links, said repeater also having the capability of modifying the serial data stream as the stream is processed by the repeater so that bidirectional communication with external equipment is possible.
The use of repeating stations, or repeaters, in data communications is well known. Such stations are required for several reasons including the need for providing bidirectional communications with external equipment. Information placed on the link and directed to an equipment must have a terminal point with which that equipment can communicate. If communication is to be bidirectional, some means of originating information to be placed on the link must also exist. The repeater can be structured to serve these functions. Often, particularly when serial transmission of data is used, this communication is achieved by changing the status of individual data bits in a message frame. As an example, if a particular external equipment desires to communicate with a control computer, the equipment may be required to change the binary status of a particular bit to set a "flag" indicating that communication is desired. The computer will subsequently respond to this indication or flag in accordance with the overall system programming. Another example of change communication is the inversion of a particular field of the message frame to indicate operational status of the link or of a particular piece of equipment on the link.
In addition to the communication function, repeaters are employed in communication data links to preserve the intelligibility of the transmitted data. This is especially true and necessary when the link is long thus resulting in high signal attenuation and increasing the probability of noise being introduced onto the link and hence into the data stream.
Prior art repeaters for digital data communication links are largely of a synchronous nature. Such repeaters tend to be relatively expensive and to require a considerable amount of time to achieve synchronism. By far the most common form of repeater of this type is that which employs a phase lock loop. Although digital phase lock loops are known, the analog form of phase lock loop is much more common than that which is customarily used. Such circuits are expensive, are much more susceptible to operational variances due to temperature, component aging, etc. than purely digital circuits and also normally require additional adjustments in the field to maintain proper operation. In addition, the phase lock loop system requires a considerable amount of time to achieve synchronism. Typically, ten to twenty data bits are used to bring the system into synchronization prior to any attempt to transmit actual data. The embodiment of this invention establishes synchronism in a fraction of a bit time.